1. Field of the Invention
The invention relates to apparatus for a bipolar active semiconductor magnetic field sensor that has a higher sensitivity than semiconductor field sensors presently existing in the art.
2. Description of the Prior Art
Over the past two decades, increasing processor sophistication along with increasingly sophisticated available application software has permitted computer systems, particularly personal computers and other microcomputer based systems, to manipulate vastly increased amounts of data than has heretofore been possible. For many years, magnetic storage media has provided an economical way to store increasingly large and substantial amounts of data. This, in turn, has fueled the growth and evolution of disk drives such that over the years disk drives have been able to provide substantially increased storage capacities at a continually decreasing cost/megabyte of stored data while keeping the overall housing of the drive at a relatively constant physical size, e.g. within a conventional "full-" or "half-" height drive bay. Clearly, the ongoing evolution of processors and accompanying application software will no doubt continue to force the development of disk drives that provide even greater storage capacities than that currently available without any increase in the physical size of the drive itself.
The capacity of a hard disk drive can be increased using either one of two methods: incorporating additional disk platters within the drive or increasing the storage capacity of each disk used within the drive. Each method possesses various shortcomings that limits its effectiveness.
First, each platter in a hard disk drive is read by at least one read head that is mounted to an end of an arm that is selectively moved in a radial direction from track to track in order to read data stored within a desired track. In a multi-platter drive, certain of these arms are situated between individual platters. In use, each arm maintains its associated read head in a position that is slightly above, but by only small fraction of a centimeter, the surface of a corresponding rotating platter. To a certain extent, these arms can be made smaller, at least in a transverse direction (i.e. the "height" of the arm as measured between adjacent disk platters), such that an increased number of platters can be stacked on a common drive spindle and housed within a single enclosure. Unfortunately, each arm which typically moves, generally on an incremental or stepped basis, at a relatively high velocity, from, for example, the innermost track to the outermost track on the disk in a few tens of milliseconds if not less, must concomitantly withstand significant physical forces in order to position the read head over a desired track in the least amount of time while ensuring that the head does not contact the surface of its corresponding platter and otherwise cause a "disk crash". Unfortunately, re-designing these arms to continually miniaturize their transverse size is a costly and tedious process and, in the absence of utilizing new and generally expensive alloys that provide increased strength and reduced weight, is also quite difficult.
Second, the storage capacity of a disk drive can be increased by either essentially using a new disk containing a different magnetic media, i.e. a magnetic coating used in a hard disk platter or a floppy diskette, that can store data at a finer resolution than that previously used along with a concomitant change in the read head(s) used within this drive to one that will support this finer resolution or, if the media on an existing disk or platter can support this finer resolution, then essentially changing the read head(s) accordingly. Developing new coatings with increasingly fine storage resolution has proven to be a tedious, extremely difficult and very expensive task. Now, assuming that the magnetic media appearing on existing platters and/or disks can itself inherently store data at a finer resolution than is currently being used by a given read head(s)--which is often the case, then changing the read head(s) to one that uses a magnetic pickup that operates at this finer resolution will impart an increased storage capacity for a disk drive that utilizes this media.
Unfortunately, using read heads having an increasingly fine resolution has been problematical. In particular, magnetic pickups, that have been traditionally used with magnetic disk drives, incorporate a coil of wire as the magnetic sensing element. While coils can be miniaturized, through thin film techniques, use of coil based pickups suffer a number of drawbacks. First, coils can not detect a static magnetic field but only flux changes. Consequently, data on a stationary disk can not be read with such a coil based pickup. For this reason, a disk must be continuously rotated whenever such a pickup is being used to read any disk stored thereon. Furthermore, merely rotating the disk is not sufficient. The rotational speed of a drive must be controlled rather accurately and within relatively fine tolerances such that the rotational speed does not vary by more than an insignificant differential amount from a nominal value either over time for any given drive or from one drive to another. In particular, since a coil detects flux changes, any variation in rotational speed will likely increase or decrease the voltages generated by the pickup from nominal values for both "1" and "0" digital levels whenever the pickup traverses over an area containing a stored data pattern. Hence, if the rotational speed of a drive either permanently lies outside of these tolerances or has a variation that exceeds these tolerances, then the data stored on a disk that has been inserted into that drive and is being read thereby may be erroneously interpreted by that drive and hence cause erroneous and/or unexpected operation of a computer that uses data supplied by that drive. Therefore, to ensure that disks are correctly read by the same or different drives, each drive must contain sufficient circuitry that provides the necessary degree of long term stable control over the rotational speed of the drive. Unfortunately, the use of such circuitry adds to the cost of the drive. Second, as the size of the coil is reduced, its sensitivity also decreases. Therefore, to obtain acceptable output signals with a sufficiently low noise content, a limit exists on the size to which the coil can be miniaturized. This limit restricts the resolution of the pickup and, in turn, limits the density of the data that can be stored on a disk that is to be read with this pickup and as well as the storage capacity of any disk drive that utilizes this pickup.
In an attempt to overcome the drawbacks associated with coil based magnetic pickups, the art teaches the use of semiconductor magnetic field sensors.
One class of such sensors is the so-called "deflection" type sensor. These sensors, characterized by that shown in illustratively U.S. Pat. Nos. 4,700,211 (issued on Oct. 13, 1987 to Popovic et al); 4,100,563 (issued on July 11, 1978 to Clark); and 3,389,230 (issued on June 18, 1968 to Hudson, Jr.) rely on establishing a quiescent single, typically majority, carrier base current in the absence of a magnetic field in a semiconductor structure that has a single emitter and two opposing collectors situated on the base. In the presence of a magnetic field applied normal to the direction of current flow, a Lorentz force imparted to the majority carriers, here electrons, will cause a portion of these carriers to be deflected and move towards one collector in lieu of the other, based upon the polarity of the field, in a direction that is itself perpendicular to the current flow and to the applied magnetic field. As such, these devices utilize the well known "Hall effect" to deflect the carriers between the two collectors. The strength of the field is proportional to the difference in the amount of current produced by each of the two collectors. Similarly, U.S. Pat. No. 3,668,439 (issued June 6, 1972 to Fujikawa et al) discloses a semiconductor device that has a single collector and two separate emitters diffused in a common base area in which a differential amount of current flows as a result of the Hall effect from the base region to the two emitters in order to forward bias one of the emitters. An output is taken from the collector that is adjacent to the forwardly biased emitter. Other deflection type sensors are illustratively shown in U.S. Pat. No. 4,163,986 (issued Aug. 7, 1979 to Vinal) and 4,129,880 (issued Dec. 12, 1978 to Vinal), both of which are assigned to the present assignee.
Hall effect sensors that also rely on carrier deflection are also described in U.S. Pat. Nos. 4,048,648 (issued to Vinal on Sept. 13, 1977--henceforth referred to as the '648 Vinal patent) and 3,997,909 (issued to Vinal on Dec. 14, 1976--henceforth referred to as the '909 Vinal patent), both of which are also assigned to the present assignee. The '909 Vinal patent describes a sensor in which a semiconductor having a sheet resistance of greater than or equal to 500 .OMEGA./square has two electrodes at opposing surfaces thereof with two spaced apart contacts situated perpendicular to the direction of single charge carriers (current) flow through the substrate. An electric field of at least 500 volts/centimeter is applied across the electrodes and hence across the substrate to establish equi-potential electric field lines and current flow within the substrate. A magnetic field that is to be detected is applied perpendicular both to the substrate and to the direction of current flow therein. The application of the external field rotates the equi-potential lines that exists within the substrate and hence causes a change in the voltage that appears between each of two contacts and ground. A similar sensor in the context of a field effect transistor (FET) device is shown in the '648 Vinal patent.
A second and different class of semiconductor magnetic sensors is the so-called "avalanche effect" type sensor. Here, these sensors characterized by that shown in U.S. Pat. Nos. 4,288,708 (issued on Sept. 8, 1981 to Vinal) and 4,276,555 (issued on June 30, 1981 to Vinal), both of which are also assigned to the present assignee, rely on use of an external magnetic field to modulate a controlled avalanche effect. Specifically, certain transistor structures exhibit sudden avalanche breakdown at a given collector-base voltage at which impact ionization occurs. The voltage at which avalanche breakdown occurs in such a structure can be bi-directionally varied by the application of an external magnetic field applied transverse to the plane of a substrate of the structure, with the direction of the variation being governed by the polarity of the applied field. Through the use of dual opposing collectors, a local avalanche voltage for one collector will increase in response to the applied field while that voltage will decrease for the other collector. As a result, a differential collector voltage will be generated that is proportional to the applied field, with the polarity of this differential voltage being dependent upon the polarity of the applied field.
With the above description in mind, one skilled in the art quickly recognizes that semiconductor magnetic sensors are capable of detecting a static field. Therefore, use of these sensors advantageously eliminates the need to accurately control the speed of a rotating disk which, in turn, can reduce the cost of a disk drive that utilizes such a sensor.
Furthermore, since semiconductor sensors only utilize integrated semiconductor structures, these sensors can be made much smaller than coil based pick-ups and thereby can detect a smaller magnetized area on a magnetic media than that destined for use a coil based pickup thereby providing an significant increase in the storage capacity of a disk used with such a sensor. In addition, semiconductor sensors are easy and rather inexpensive to fabricate.
Unfortunately, known semiconductor magnetic sensors, whether of the "deflection" or "avalanche effect" type, suffer from one or more drawbacks. "Deflection" type sensors exhibit a very low sensitivity to, a detected magnetic field. Although "avalanche effect" type sensors possess increased sensitivities over that provided by "deflection" type sensors, "avalanche effect" sensors disadvantageously suffer from other drawbacks. First these sensors exhibit avalanche induced noise. Second, the avalanche phenomena is highly non-linear and, for that reason, renders any device, such as a sensor, that relies on its use very difficult to control. Third, once a local avalanche occurs, such as in the vicinity of one collector, that avalanche must be quenched prior to the next use of the sensor. As such, an "avalanche effect" type sensor requires a finite amount of recovery time that disadvantageously limits the response and bandwidth of the sensor.
Given these drawbacks, one can readily appreciate that use of a "deflection" type sensor would be favored over use of an "avalanche effect" type sensor. However, since "deflection" type sensors known in the art are not sufficiently sensitive to a detected magnetic field, coil based pickups continue to be widely used in the art even in view of the finer detection resolution available through semiconductor sensors.
Thus a need exists in the art for a semiconductor magnetic field sensor that possesses a higher sensitivity than do magnetic sensors heretofore known in the art, specifically both coil and semiconductor types. Use of such a sensor in a read head of a disk drive, with a disk that is capable of storing data at a finer resolution over that at which it is presently being used with a coil type pickup, would advantageously enable that the disk to store data at an increased density over that obtainable with a coil type pickup and thereby substantially increase the storage capacity of the drive. In addition, since such a sensor is capable of detecting a static field and thereby eliminates the need for accurate speed control of the disk and would also be rather inexpensive to fabricate, use of such a sensor might likely reduce the manufacturing cost of the drive.